1. Field of the Invention
The present invention relates generally to display systems and, more particularly, to display systems employing a video display device and a hard copy device which produces a hard copy of the data shown by the video display device screen. The invention may be particularly advantageous where both graphic and alphanumeric data are displayed by the video display device and reproduced by the hard copy device.
2. Description of the Prior Art
Various display systems are known where a video display device is first used to manipulate the data displayed on the screen of the device and, once the data is satisfactorily arranged or otherwise processed, the data is then printed by a printer. Such systems allow corrections to be easily made in the displayed data and can provide a user with the ability to view numerous alternatives before a final printed copy is produced. Because the visual representation produced by the video display screen can be repeatedly and quickly modified, the relatively more time consuming process of producing the hard copy can be reserved for instances where the hard copy simply preserves the final desired data as seen on the screen of the display device. Examples of systems of this nature include word processing systems and computer aided design systems.
Where alphanumeric characters alone are to be copied by the hard copy device from the display device screen, it is often desirable that the alphanumeric characters produced by the hard copy device closely resemble the characters shown on the screen. This may be particularly useful where, for example, a particular character font is in use or where the alphanumeric characters include special or unique characters. Generally it has been the practice to employ a dedicated character generator within the hard copy device to generate the alphanumeric characters produced thereby. Where this is the case, the hard copy device character generator must exactly match the character generator within the video display device. Such a requirement can increase the costs of a video display system by requiring a non-standard character generator to be used in the hard copy device.
Where the video display screens in such systems are adapted to show composite displays of alphanumeric characters and symbols along with graphics, it is necessary that the hard copy device produce a composite copy of the entire screen. Several systems for solving this problem have been previously used.
For example, the entire screen of the display device may be considered a matrix of picture elements (pixels) which are turned on or off. By turning on or off selected ones of the pixels, a matrix image is defined which can include both graphics and alphanumeric characters. In such an instance, a hard copy device need only reproduce the pixels on a one-for-one basis to thereby copy the composite graphic and alphanumeric display.
With such systems, however, it is difficult to manipulate only the graphics portion of the image or only the alphanumeric characters. For example, if it is desired to erase and replace the graphics portion of the display image, the entire display image must be cleared. Then, any alphanumeric characters must again be written into the pixel matrix in addition to the new graphics, thus requiring additional processing time. On the other hand, similar difficulties are encountered if only the alphanumeric characters are to be replaced or varied. In this instance, the graphics portion of the display image must be redrawn, creating a particular difficulty where the data necessary for the graphics display image is no longer available.
An alternative solution to this problem is essentially the converse of the first solution. That is, instead of treating the entire video display screen as a matrix of individually selectable pixels, the display image or screen is defined as a matrix of characters each comprising a smaller matrix or cell of pixels. The characters can be alphanumeric characters or graphic elements used to form a portion of an overall graphic image. To then reproduce the video display image, the hard copy device reproduces the characters displayed on the screen without regard as to whether the character is alphanumeric or is a graphic element. With systems of this nature, however, the graphics characters are often extremely limited and are not suitable for applications requiring high resolution graphics. In particular, the graphics characters are generally formed by turning on or off blocks of pixels within the character cell. For example, one such graphics character may be formed by turning on the pixels in the upper half of the character cell while turning off the pixels in the lower half. Because blocks or groups of pixels are controlled within the character cell, high resolution graphics requiring control of individual pixels on the video display screen cannot be produced.
Yet a third solution to the above-identified problem is to use separate memories to store the alphanumeric characters and the graphic portion of the display image yet simultaneously display them on the display device screen. When the image shown on the screen is to be copied by the hard copy device, the device then first reproduces the graphics memory and then reproduces the alphanumeric character memory. This type of system typically employs a plotter as a hard copy device and is not suited for applications employing a dot matrix printer. This distinction exists because the plotter is able to return to a portion of the hard copy as often as is necessary in order to produce a composite copy of the alphanumeric characters and graphics. Dot matrix printers, on the other hand, are commonly designed to pass over each portion of the hard copy only once, thus making it substantially impossible to accurately return to a previously printed portion of the hard copy. Although a plotter can produce a composite image in systems of this type, plotters are generally slower and more costly than dot matrix printers.
Thus, there is a need for a video display system which can accurately reproduce the characters shown on a video display device screen. Moreover, there is a need for such a system employing a dot matrix type hard copy device that can copy a composite display of alphanumeric characters and graphics. Furthermore, it is desirable that such a system allows the displayed alphanumeric characters and graphics to be separately manipulated. The present invention overcomes the limitations described above and meets these needs.